Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers. At this wavelength, nearly all known solid materials absorb a significant fraction of EUV light passing through the material. Thus, one generated, EUV light must be transmitted through vacuum of gas and reflected (since refracting lenses are generally unavailable) by mirrors (e.g. grazing incidence or near normal incidence multi-layer mirrors) along the entire path from the point of generation to the workpiece requiring exposure (e.g. wafer, flat panel, etc.)
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. These elements can include, but are not limited to xenon, tin, water, lithium.
In one such method, often termed laser-produced-plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. In some cases, other suitable energy beam (e.g. electron beam) may be used in place of the laser. In another method, often termed electric discharge-produced-plasma (“DPP”), the plasma may be produced by disposing a material having the required line-emitting element between a pair of electrodes and then generating an electrical discharge between the electrodes.
In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source element, such as xenon (Xe), tin (Sn) or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) having an ellipsoidal shape is positioned at a distance from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. In a typical setup, the EUV light must travel within the light source about 1-2 m from the plasma to the intermediate location, and as a consequence, it may be advantageous, in certain circumstances, to limit the atmosphere in the light source chamber to gases having relatively low absorptance of in-band EUV light.
For EUV light sources designed for use in high volume manufacturing (HVM) environments, e.g. exposing 100 wafers per hour or more, the lifetime of the collector mirror can be a critical parameter affecting efficiency, downtime, and ultimately, cost. During operation, debris are generated as a by-product of the plasma which can degrade the collector mirror surface and other optics. These debris can be in the form of high-energy ions, neutral atoms and clusters of target material. Of these three types of debris, the most hazardous for the collector mirror coating is typically the ion flux. In the absence of debris mitigation and/or collector cleaning techniques, the deposition of target materials and contaminants, as well as sputtering of the collector multilayer coating and implantation of incident particles can reduce the reflectivity of the mirror substantially. In this regard, co-pending, co-owned U.S. patent application Ser. No. 11/786,145 filed on Apr. 10, 2007, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, (the contents of which are hereby incorporated by reference herein) discloses a device in which a flowing buffer gas such as hydrogen at pressures at or above about 100 mTorr is used in the chamber to slow ions in the plasma to below about 30 eV before the ions reach the collector mirror, which is typically located about 15 cm from the plasma.
It is currently envisioned that about 100 W of EUV power, or more, will need to be delivered to a scanner/stepper to allow for efficient high volume EUV photolithography. To obtain this output power, a 5-20 kW drive laser, e.g. CO2 laser, may be used to irradiate a source material such as a stream of tin droplets. Of the 5-20 kW of power delivered within the EUV light source chamber, calculations indicate that about 20%-80% of this power may be transferred to a buffer gas in the chamber.
Unlike the relatively harsh environment of the light source (which as indicated above may include debris, source material vapor and compounds, cleaning etchants such as HBr, ion slowing buffer gas(es) such as hydrogen, (which may be at relatively high pressures and/or relatively high flow rates), heat, etc.,) the environment within the stepper/scanner is typically more benign. Indeed, within the chamber of a stepper/scanner (which typically includes complex optics to establish illumination, patterning and projection as well as complex mechanical arrangements to move the wafer stage relative to the patterning optics, e.g. reticle), a near vacuum environment which is nearly completely free of the debris, gas(es), pressures and/or heat which may be found in the light source chamber, is desirable. However, as indicated above, solid, non EUV absorbing materials are unavailable to establish a suitable barrier between, e.g. a light source chamber and scanner optics chamber, and, as a consequence, more complex arrangements must be developed to separate these environments while still passing EUV light from one chamber to another.
With the above in mind, applicants disclose systems for managing gas flow between chambers of an extreme ultraviolet (EUV) photolithography apparatus, and corresponding methods of use.